By default, when you display or play through a sequence in the Viewer, Nuke only calculates and caches the scanlines for the visible area. It's not caching the full frame, but a subset of the scanlines you're viewing. For example, if you have set the zoom level to be ÷4, Nuke is only caching 1/4 of the scanlines that make up the frame. In a lot of cases, this allows for faster playback and may be what you want.
However, if you play through a sequence and then pan around or change the zoom level, Nuke has to calculate the scanlines it didn’t calculate before. This may not be what you want if you are performing tasks that require you to constantly pan around or zoom in and out of the plate (such as paint and roto). If this is the case, you can click on the full frame processing button in the controls at the top of the Viewer to force Nuke to process every scanline in the image. Compared to the default mode, this may take slightly longer initially and requires more space in the Viewer cache, but once Nuke has cached the frames you require, you can pan around and change the zoom level without breaking the cache or affecting playback.